Image formation based upon an electrophotographic method is generally performed by a process which includes forming an electrostatic image on a photoconductor (electrostatic image bearing member), developing the electrostatic image with a developer so as to form a visible image (toner image), transferring the visible image onto a recording medium such as paper, and fixing the transferred visible image to the recording medium with application of heat, pressure, a solvent gas, etc. so as to obtain a fixed image (refer to PTL 1).
Regarding the developer, one-component developers for which magnetic toners or nonmagnetic toners are solely used, and two-component developers composed of toners and carriers are known. One-component developing methods are classified into magnetic one-component developing methods and nonmagnetic one-component developing methods, depending upon whether or not magnetic force is used to keep toner particles on a developing roller. As for the toners, each toner is generally produced by a kneading pulverization method in which a thermoplastic resin is melt-kneaded along with a colorant, etc., then finely pulverized and classified. Additionally, in some cases, inorganic fine particles or organic fine particles are added to surfaces of toner particles according to necessity, for the purpose of improving the fluidity and cleanability of the toner particles.
The toner obtained by the kneading pulverization method is generally fixed by being heated and melted with the use of a heat roll. In doing so, when the temperature of the heat roll is too high, hot offset may arise in which the toner melts excessively and fuses with the heat roll; conversely, when the temperature of the heat roll is too low, the toner does not sufficiently melt, and thus the fixation of the toner may be insufficient. In recent years, in view of energy saving and size reduction of apparatuses such as copiers, toners that achieve a favorable balance between hot offset resistance and low-temperature fixability, allowing the temperature at which hot offset arises to be higher and reducing the fixation temperature, have been demanded. Especially with regard to full-color copiers, full-color printers and the like, toners having lower melting points are demanded, since the glossiness and color mixture of images produced are important; however, the toners having low melting points easily cause hot offset and are inferior in terms of heat-resistant storage stability in a high-temperature and high-humidity environment. Accordingly, a conventional full-color apparatus employs a method of applying silicone oil or the like to a heat roll so as to provide toner releasability.
However, this method requires an oil tank, an oil applying device and the like, which leads to complexity and enlargement of an image forming apparatus. Moreover, since the thermal roll easily degrades, regular maintenance is required. Further, there is a problem in which the oil is attached to a recording medium such as copy paper or OHP film, thereby causing the color tone of images to degrade.
Accordingly, the method of providing toner releasability without application of oil to a heat roll, and adding a release agent such as a wax to a toner for preventing the problem of fusion of the toner is generally employed. Here, the toner releasability is greatly affected by the dispersed state of the wax in the toner. If the wax is compatible with a binder of the toner, toner releasability cannot be sufficiently exhibited. In the case where the wax is incompatible with the binder, the wax can exist as domain particles, thereby exhibiting toner releasability. On this occasion, if the dispersion diameter of the domain particles is too large, the proportion of the wax present in the vicinities of the surfaces of toner particles relatively increases; thus, the domain particles may aggregate, causing degradation of particle fluidity, the wax or a carrier may transfer to a photoconductor, etc. during long-term use, causing filming, and so it may be impossible to obtain images of favorable quality. If the dispersion diameter of the domain particles is too small, the wax is finely dispersed to excess and thus adequate toner releasability may not be yielded.
In the kneading pulverization method, since it is difficult to control the dispersion diameter of the domain particles and the wax is liable to be present on fracture surfaces, the amount of the wax exposed at the toner surface is large and so the above problems such as degradation of particle fluidity and occurrence of filming may arise. Further, there exist the following problems: the toner obtained by the kneading pulverization method generally has a wide particle size distribution, varies in frictional chargeability and easily causes fogging and the like; also, it is difficult to obtain a small-particle-diameter toner (2 μm to 8 μm in volume average particle diameter) for reasons related to production efficiency, and the demand for improvement in image quality can hardly be met.
Accordingly, note is taken of toners obtainable by granulation in an aqueous phase. The toners have narrow particle size distributions, can be easily reduced in particle diameter, make it possible to obtain high-quality, high-definition images, and are superior in offset resistance and low-temperature fixability due to high dispersion of a release agent such as a wax. Also, the toners are superior in transferability due to their uniform chargeability, and favorable in terms of fluidity, which gives an advantage in terms of design of a developing device (for example, it is possible to design a hopper with more freedom and reduce the toque with which a developing roll is rotated).
As the toners obtainable by granulation in an aqueous phase, toners obtainable by a suspension polymerization method or an emulsion polymerization aggregation method (hereinafter referred to also as “chemical toners”) have been conventionally developed.
The suspension polymerization method is a method of obtaining toner particles by adding a monomer, a polymerization initiator, a colorant, a wax, etc. into an aqueous phase containing a dispersion stabilizer with agitation so as to form oil droplets, and then increasing the temperature to effect a polymerization reaction. The suspension polymerization method can achieve reduction in the diameter of the toner particles. Regarding the suspension polymerization method, it is difficult to make the wax appropriately present at the surfaces of the toner particles unless a dispersion stabilizer is used, because the wax tends to enter the oil droplets easily when the oil droplets are being formed; here, there is a problem in which if the dispersion stabilizer remains, it causes a decrease in chargeability.
As the emulsion polymerization aggregation method, there is, for example, a method proposed in which a polyester resin is used as a binder resin; fine particles obtained by subjecting the polyester resin to emulsion dispersion in an aqueous phase and then removing the solvent are aggregated with a dispersion formed by dispersing a colorant, a release agent (wax), etc. in an aqueous phase; and the aggregated matter is heated and fused so as to produce toner particles (refer to PTL 2 and PTL 3). According to this method, since ultrafine particles are not generated, there is no loss of emulsification, and further, it is possible to produce a toner having a sharp particle size distribution without needing classification. However, when the fine particles obtained after the solvent removal are aggregated, mere aggregation of the fine particles leads to insufficient unification thereof, thereby creating cracks or the like at interfaces after the unification. Therefore, a heating step for allowing the unification of the particles to proceed by heat is necessary.
However, when the heating is carried out, blooming of a wax component finely dispersed in the toner particles may arise (the wax component may be deposited on the surfaces), and/or aggregation, etc. of finely dispersed particles of the wax may arise, thereby making it impossible to maintain the state in which the wax is finely dispersed in a sufficient manner. Especially in the case where a wax having a low melting point is used, it easily melts in the heating step, and thus there is a problem in which favorable toner releasability cannot be secured and so there is a lack of suitability of the toner for oilless toner fixation with a heat roll.
Meanwhile, there has been proposed a method in which wax fine particles covered or impregnated with a vinyl polymer by adding a polymerizable vinyl monomer and a water-soluble polymerization initiator to a wax emulsion to effect polymerization are added to a toner composition when the toner composition is emulsified, and the wax fine particles are thereby uniformly and firmly attached to the toner surface (refer to PTL 4).
However, this method requires polymerization of a wax emulsion and a polymerizable vinyl monomer; moreover, the glass transition temperature (Tg) of a resin contained in the wax fine particles is high; thus, there is a problem in which the toner is inferior in low-temperature fixability and releasability at low temperatures.
Meanwhile, there has been proposed a method in which a polymerizable monomer that contains a polar group-containing substance and a wax is subjected to suspension polymerization in water to produce a toner, and thus the toner contains a wax having a low melting point that is unable to be used for a toner produced by a pulverization method (refer to PTL 5). In this method, a pseudo-capsule structure is employed in which a nonpolar component such as a wax is not present in the vicinities of the surfaces of toner particles, as opposed to a polar component, but covered with the polar component at the surfaces.
However, the dispersion of the wax inside the toner particles is not analyzed and is therefore unknown.
Meanwhile, use of a toner has been proposed in which the amount of a wax contained therein is in the range of 0.1% by mass to 40% by mass, and the wax exposed at the toner surface accounts for 1% by mass to 10% by mass of the constituent compounds exposed at the toner surface (refer to PTL 6). The proportion of the wax exposed at the toner surface is measured by ESCA and thus determined.
However, analysis based upon ESCA is only possible within approximately 0.1 μm in depth from the outermost surface of the toner, and thus it is difficult to know the dispersed state of the wax which lies further inside and suitably exhibits toner releasability in a fixing step.
Meanwhile, use of a toner has been proposed in which a wax is encapsulated in toner particles and is locally present at the surfaces of the toner particles (refer to PTL 7). However, details of the dispersed state of the wax in the vicinity of the toner surface are unknown.
Meanwhile, a method has been proposed in which the proportion of a wax exposed at the toner surface is measured by FTIR-ATR and thus determined (refer to JP-A No. PTL 8). However, there is a complete trade-off between blocking resistance of the toner and hot offset resistance of the toner, and between prevention of filming and prevention of wrapping of paper. Merely improving properties of the toner and controlling the dispersed state of the wax does not suffice to improve fixability of the toner further.
Therefore, there is a strong demand for a method for stably and efficiently obtaining a toner capable of maintaining the advantages of the chemical toners (i.e., a small particle diameter, a narrow particle size distribution and superior fluidity), yielding superior releasability at low temperatures, lessening the occurrence of filming, securing a favorable balance between low-temperature fixability and heat-resistant storage stability, and thus forming high-quality images. However, such a method has not yet beet provided in reality.
Generally, for fixation of toner, a method of directly pressing a fixing member (such as a fixing roller or a fixing belt) against an unfixed image so as to thermally melt the toner and fix the melted toner to an image bearing member (such as paper), in other words a thermal pressing fixing method, is preferably employed in view of thermal efficiency, simplicity of a fixing mechanism, production costs of the fixing member, etc.
FIG. 1 is an explanatory drawing of a belt-type fixing device (denoted by the letter Z in the drawing). As shown in FIG. 1, this fixing device includes a fixing belt B provided in a rotatable manner by means of a heating roller R3 and a fixing roller R1. The fixing belt B touches a cleaning roller R4 between the heating roller R3 and the fixing roller R1. The fixing roller R1 includes a core metal and a heat-resistant sponge rubber layer on the outer circumference of the core metal. The heating roller R3 includes a metal core which houses a heat source H such as a halogen lamp, and the fixing belt B is heated from inside with the radiant heat of the heat source H. The fixing device also includes a pressurizing roller R2 provided in such a manner as to touch the fixing roller R1 with the fixing belt B situated in between. By means of a pressurizing spring P, the pressurizing roller R2 pressurizes the fixing roller R1 and provides tension to the fixing belt B. Also, the pressurizing roller 2 is rotated by a driving unit (not shown), and this causes the fixing roller R1 to rotate depending upon the rotation of the pressurizing roller R2. In such a belt-type fixing device, transfer paper is passed along a guide G through the part between the fixing belt B heated by the heating roller R3 and the pressurizing roller R2, and toner attached onto the transfer paper is pressurized by the pressurizing roller R2 while softened by the heat of the fixing belt B, and thus fixed onto the transfer paper.
A belt-type fixing device utilizing electromagnetic induction heating includes a fixing roller, an opposed roller placed in parallel with the fixing roller and made of a nonmagnetic material, a fixing belt in the form of an endless belt placed in a winding manner between the fixing roller and the opposed roller, an induction coil which heats the fixing belt from outside, and a pressurizing roller which presses the fixing roller with the fixing belt situated in between. Recording paper is passed between the fixing belt and the pressurizing roller; at this time, unfixed toner on the recording paper is fixed thereto by the heat from the fixing belt and the pressing force of the pressurizing roller (refer to PTL 9). As shown in cross section in FIG. 2, a fixing belt (denoted by the letter C in the drawing) generally has a laminated structure in which a base material 1, a heat generating layer 2, an elastic layer 3 and a release layer 4 are laid in this order from the bottom to the top.
The base material 1 is in the form of an endless belt made of a heat-resistant resin. Examples of the material for this heat-resistant resin include polyimides, polyamideimides and polyether ketones (PEEK). The thickness of the base material 1 is generally set at 20 μm to 100 μm in view of the rigidity and heat capacity of the fixing belt.
For the heat generating layer 2, a metal such as SUS, iron, nickel, manganese, titanium, chromium or copper is used. The elastic layer 3 is necessary to yield uniformity of images, and a heat-resistant rubber (approximately 100 μm to approximately 300 μm in thickness) such as silicone rubber or fluorine rubber is used therefor. The release layer 4 is formed of a fluorine rein, etc. superior in heat resistance and durability, in view of its contact under pressure with transfer paper and toner.
However, in the above-mentioned conventional fixing device, the fixing belt is merely heated by the induction coil and the temperature of the fixing belt is not controlled. Thus, hot offset easily arises at both ends of the belt. Specifically, when recording paper of small size is continuously fed, both ends of the belt are not deprived of heat by the recording paper and thus increase in temperature; in this state, when recording paper of large size is fed, there is a problem in which hot offset arises at both ends of the belt.
Also, in the conventional fixing device, ends of the opposed roller have large heat capacity owing to the presence of bearings, etc. at the ends. Thus, when the fixing belt has started being heated by the induction coil, the heat travels toward the ends of the opposed roller, and the temperature increase rate of the ends of the opposed roller is lower than that of the center of the opposed roller as shown in FIG. 3. Consequently, there is a problem in which the time spent until the fixing device becomes usable, namely the rising time, lengthens.
Meanwhile, there has been proposed a fixing device including a fixing belt which endlessly moves while supported by a heating roller and a fixing roller with a small belt curvature and heated by the heating roller, wherein the fixing belt is pressed against a toner image on a transfer material so as to heat and fix the toner image on the transfer material (refer to PTL 10). This fixing belt generally has a three-layer structure composed of a substrate made of a heat-resistant resin (such as a polyimide) or metal, an elastic layer made of a heat-resistant rubber or elastomer, and a release layer (outermost layer) made of a fluorine resin. The release layer made of a fluorine resin is formed by covering the elastic layer with a fluorine resin tube (formed by extrusion molding) and then heating and melting (hereinafter referred to also as “firing”) the fluorine resin. Alternatively, the release layer is formed by applying fluorine resin particles over the elastic layer by means of a spray, etc. and then firing the fluorine resin. As just described, by forming the release layer of a fluorine resin, the fixing belt can be superior in toner releasability and heat resistance. The fixing belt yields great effects, especially in terms of toner releasability, and is therefore effective against hot offset of toner and wrapping of paper.
However, the fluorine resin is poor in bendability, so that when the fixing belt is used for a long period of time, supported by the heating roller and the fixing roller with a small belt curvature, there is a problem in which cracks are created in the release layer and thus sufficient durability of the belt cannot be secured.
Examinations of fixing mechanisms have been carried out (refer to NPL 1). Such examinations and proposals of fixing mechanisms alone do not lead to a fundamental solution to the problems for reasons similar to the above reasons.
Nowadays, application of electrophotographic image forming methods to fields of printing with high image areas and at high speed, such as offset printing, is becoming common. Here, fixation of an image to an image transfer medium with the lowest possible energy is an objective of the electrophotographic image forming methods. Meanwhile, regarding toners for use in image formation, it is important that the fixation temperature of the toners themselves be reduced and that hot offset at high temperatures be prevented. Accordingly, there has been a proposal to reduce the fixation temperature by using a polyester resin that is advantageous in terms of low-temperature fixation. Also, as methods for preventing hot offset, the following methods are well known: a method of controlling the viscoelasticity of a toner by introducing a resinous polymer into the toner; and a method of suppressing the viscoelasticity of a toner by enhancing the releasability of the toner from a fixing member with the use of a release agent such as a wax.
Regarding the use of a wax, use of a paraffin wax has been proposed (refer to PTL 11); further, definition of the range of melting points of a wax in accordance with the DSC method has been proposed. In many such proposals, effects on toner releasability have been confirmed. Here, as described above, high image quality which does not differ from the initial image quality (even when printing is carried out in large amounts with a high image area) is required in the field of high-speed printing.
In the case where a conventionally proposed wax is used in an electrophotographic image forming apparatus which conducts printing in large amounts, it has been proved that a paraffin wax, which is highly volatile, causes troubles such as smearing of members of the image forming apparatus and smearing of transfer media themselves.
For example, it has been proposed that by determining the heating loss at 220° C., favorable effects can be exhibited in securing storage stability and preventing a spent carrier and filming over a photoconductor (refer to PTL 12). However, even when the requirements of the heating loss at this temperature are not satisfied, the above-mentioned troubles may not arise in the case of a toner producing method using a type of wax and an aqueous medium. If anything, it has been proved that even when the requirements of the heating loss are satisfied, the prevention of smearing of members may be insufficient in high-speed printing, and the separability of transfer media may also be insufficient at the time of high-speed printing. Also, it has been proved that when the requirements of the heating loss are not satisfied, favorable effects on prevention of smearing of the members can be yielded by satisfying the claims of the present application. Meanwhile, in the case where a paraffin wax having a high melting point is merely used, it is difficult to secure desired toner releasability, thereby possibly causing hot offset and/or decreasing image quality (e.g., decreasing glossiness). In reality, merely determining the melting point of the paraffin wax does not suffice to prevent smearing inside a machine or secure desired toner fixability.
Also, images produced by high-speed printing are, in most cases, full-color images with high image area ratios. In cases where a heating medium and a transfer medium need to be separated from each other at high speed and surely in a fixing step, it is very important to achieve a favorable balance between securement of toner releasability with the use of a wax and prevention of smearing inside a machine.
Meanwhile, there has been a proposal to remove nonuniformity of images caused at the time of fixation and thereby increase image quality, by using a microcrystalline wax (refer to PTL 13). To remove nonuniformity of images, the endothermic peak of the wax and the half width of the endothermic peak are defined. Although this makes it possible to remove nonuniformity of images, the wax has a high melting point, which is disadvantageous to low-temperature fixation. Meanwhile, merely lowering the endothermic peak of the wax in view of low-temperature fixability leaves a problem concerning separability between paper and roller(s) at high temperatures.
As just described, in reality, further improvement is required to secure a favorable balance between low-temperature fixability and heat-resistant storage stability and a favorable balance between low-temperature fixability and separability of paper from roller(s) at high temperatures, reduce the volatile matter content at the time of fixation and thus obtain high-quality images.